1. Field of the Invention
The present invention relates to an apparatus for the print throw-on and throw-off of the blanket cylinder in the printing unit of a sheet fed offset printing machine and a method for controlling the operation of the apparatus.
2. Discussion of the Related Art
In sheet-fed offset printing machines, it is known to mount the rubber blanket cylinder on two journals, each having eccentric bearings pivotable in the walls of the frame of the printing machine. As a result of the pivoting of the eccentric bearings, the rubber blanket cylinder can be thrown off from the sheet-guiding impression cylinder. In addition, as a result of a further pivoting movement of the eccentric bearings, a throw-off of the rubber blanket cylinder from the plate cylinder carrying the printing form takes place. For the print throw-on, the above described operation is executed in reverse order, that is to say, as a result of a first pivoting movement the rubber blanket cylinder is thrown onto the plate cylinder and then as a result of a second pivoting movement, the rubber blanket cylinder is thrown onto the impression cylinder. As a rule, the throw-on and throw-off of the rubber blanket cylinder takes place during channel correspondence, that is to say, when the channel of the rubber blanket cylinder is opposite the channel of the impression or plate cylinder.
In order to initiate the above described pivoting movements of the eccentric bearings of a rubber blanket cylinder, particularly in the case of high speed sheet-fed offset printing machines, it is known to use actuating means capable of being loaded with pressure medium, for example, pneumatic devices. DE 4,013,075 C1 discloses a print throw-on and throw-off apparatus of this type, in which the piston rods of two pneumatic cylinders are articulated on a toggle mechanism via a ternary member and the toggle mechanism acts on a move-off shaft extending between the side walls of the frame of the printing machine. The pivoting of the two eccentric bearings takes place in each case via levers attached to one end of the move-off shaft, with a link being interposed in each case. The counterbearing of the toggle is adjustable for the purpose of print regulation which is dependent on the thickness of the printing material. On account of the characteristic of the toggle mechanism in conjunction with the articulation of the pneumatic cylinders, it is necessary to activate both cylinders, that is to say initiate the entire pivoting travel of the eccentric bearings, in order to throw off the rubber blanket cylinder from the plate cylinder.
A further print throw-on and print throw-off apparatus, which uses actuating means capable of being loaded by pressure medium is disclosed in DE 4,142,755 A1. In this apparatus, there is provided a coupling of the rubber blanket cylinder and plate cylinder mounting dependent on the thickness of the printing material. In this apparatus, a variation in the setting of the thickness of the printing material causes no variation in the setting between the plate cylinder and the rubber blanket cylinder (printing with and without bearer-ring contact). However, a disadvantage associated with this apparatus is its relatively complicated construction.
In very high-speed sheet-fed offset printing machines (15,000 prints per hour or more), it is the throw-off movements described above which have to be executed in the shortest possible time by pivoting of the eccentric bearings. However, the pivoting movement of the eccentric bearings gives rise to a torque jolt which is transmitted via the gear train to the remaining printing units. If, for example, the print is thrown off in the first printing unit, this torque jolt generates a disturbance in the subsequent printing units, so that, as a result of mackling caused by the jolt, e.g., blurring or double impression, the sheets printed there are poor quality prints that have to be discarded. A similar effect occurs during the print throw-on, since in this case, for example, during the throw-on of the last printing unit, the disturbance caused thereby likewise causes mackling in the preceding printing units which are already thrown on.
To solve the above described problem, it has been proposed in DE 4,206,626 A1 and DE 4,206,627 A1, to execute the throw-on and throw-off of the rubber blanket cylinder in a manner uncoupled, i.e., essentially reactionlessly, from the drive of the machine. However, the manner in which this operation is to be achieved is not specified in these publications.